[doc] Improve operators page (#4073)
[doc] Improve operators page (#4073)

* [doc] Improve operators page

* add conditional expression

* same type

* Update docs/lang/articles/basic/operator.md

Co-authored-by: Vissidarte-Herman <bb38b48046b6557306e3b6798e1a0188f4c150c8@users.noreply.github.com>

* add abs and pow

* apply suggestions

* split cpu and cuda

Co-authored-by: Vissidarte-Herman <bb38b48046b6557306e3b6798e1a0188f4c150c8@users.noreply.github.com>
yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi